Fixed #63 - `use_directory_urls` option has no effect.

- The `use_directory_urls` option is now passed forward to the
SiteNavigation object in order to correctly generate links to the
target file when needed.
- The post-processing of the HTML generated by converting from Markdown
now takes the `use_directory_urls` option passed to the SiteNavigation
object into account when changing anchor targets.
- The dev server mode (mkdocs serve) still ignores the
`use_directory_urls` option in order to have user-friendly URLs.
- There is a new test case for post-processing converted Markdown when
  `use_directory_urls` is `False`.
Fixes #65 - Links to other files within documentation are broken on Python 2.6

There was a bug in `posixpath.relpath()` in this version of Python. The
bug was worked around by a verification of the corner case were the
`start` argument is the root directory.
Fixes indentation issues which caused lint errors.

Those errors were causing the `runtests` script to fail.
Fixes location of static assets.

The previous commit made `base_url` incorrectly empty.
Merge pull request #64 from thiagomael/master

Fixed issue #63 - `use_directory_urls` option has no effect.